Is that a whole UI re-write in a native/non-web framework? Because in case that's not, this is still running in a browser, i.e. it stands at 80% or so of the bloat factor of electron, and what makes it faster is possibly (mainly?) that they haven't gotten yet to adding as much stuff as seen on the regular client.

Discord is a server-side protocol (the client isn't doing much besides displaying the server-prescribed state), there's very little logic/computation going on on there that should explain a world of difference.